A well-known issue with using switching mode power supply is its relative high operation switching frequency. This high frequency signal is coupled back into the AC mains input and becomes a component of the AC mains that can cause noise problems for those devices connected to the same AC mains power line. In addition, the high frequency signals are also radiated by the power supply as electromagnetic waves to create Electromagnetic Interference (EMI) that can also cause problems for communications devices in the neighborhood of the power supply.
The common practice to alleviate the problem of EMI is by introduction of frequency jittering to spread the switching frequency over a wider bandwidth which can reduce the peak value of the EMI generated by the power supply at each frequency. One of examples of frequency jittering EMI reduction scheme is illustrated in FIG. 1 prior art. A Frequency Jittering device comprises a current control charge/discharge type oscillator 111, 7 bit digital counter 140 and Digital to Analog (D/A) converter 150. The oscillator 111 includes a capacitor 134 which is periodically charged and discharged. A hysteresis comparator 136 is used to detect the voltage level of the capacitor 134 and produces an output signal 101 which controls the charge and discharge of the capacitor 134 through charging and discharging circuit paths. The 7-bit counter 140 is then clocked by the oscillator output signal 101. The counter 140 outputs driver the D/A converter 150, whose output 113 is connected to the control input of the oscillator 111 for varying the oscillation frequency. The merit of this frequency jittering scheme is the elimination of an expensive and bulky EMI filter but its draw back is the rise of average noise floor which is not acceptable in certain application like in high fidelity audio system. For a high fidelity audio system, it can tolerate higher averaging output noise floor generated from switching power supply at high sound volume but becomes very noise sensitive at light sound volume.
Accordingly, a new frequency jittering scheme that can apply low cost simple EMI filter for EMI reduction but also can keep low level of switching power supply output noise floor at light load conditions is needed.